---
title: Multi-thread Backtrace for Silently Hanging Tests
teaser: Tests hanging with no feedback on how to solve the problem? Try this!
tags: rspec,minitest,debugging
author: Aji Slater
published_on: 2023-09-20
---

Scenario: You're running your test suite. There are hundreds, if not thousands,
of tests and assertions running in a new random order. Green dots fill your
screen until... they don't.

No new red dots, but no new green dots either.

Your tests are hanging. No amount of waiting sees them finish.

But without any feedback, how are you to know what to debug? How will you know
what test is even causing the problem?

Solution: Put this snippet from [@myronmarston][1] into your `test/test_helper.rb`
or `spec/spec_helper.rb`.

```ruby
puts "test run pid: #{Process.pid}"

trap 'USR1' do
  threads = Thread.list

  puts
  puts "=" * 80
  puts "Received USR1 signal; printing all #{threads.count} thread backtraces."

  threads.each do |thr|
    description = thr == Thread.main ? "Main thread" : thr.inspect
    puts
    puts "#{description} backtrace: "
    puts thr.backtrace.join("\n")
  end

  puts "=" * 80
end
```

([original appearance of snippet: April 14, 2015 (Github)][2])

Then when the tests hang, kill the process with `kill -USR1 <pid>` and voila!
Backtraces (for every thread if it's parallelized)!

Your terminal should look something like this. (filepaths truncated for
legibility)

```
================================================================================
Received USR1 signal; printing all 14 thread backtraces.

Main thread backtrace:
...dev/project/test/test_helper.rb:348:in `backtrace'
...dev/project/test/test_helper.rb:348:in `block (2 levels) in <main>'
...project/test/test_helper.rb:344:in `each'
...project/test/test_helper.rb:344:in `block in <main>'
.../active_support/testing/parallelization/server.rb:59:in `sleep'
.../active_support/testing/parallelization/server.rb:59:in `shutdown'
.../active_support/testing/parallelization.rb:50:in `shutdown'
.../active_support/testing/parallelize_executor.rb:26:in `shutdown'
.../minitest-5.18.0/lib/minitest.rb:163:in `run'
.../minitest-5.18.0/lib/minitest.rb:83:in `block in autorun'

#<Thread:0x000000010a3953e0@DEBUGGER__::SESSION@server \
   .../debug/session.rb:154 sleep_forever> backtrace:
<internal:thread_sync>:18:in `pop'
.../gems/debug-1.6.3/lib/debug/session.rb:199:in `pop_event'
.../gems/debug-1.6.3/lib/debug/session.rb:203:in `session_server_main'
.../gems/debug-1.6.3/lib/debug/session.rb:174:in `block in activate'

#<Thread:0x000000010c6f9db8 .../connection_pool/reaper.rb:40 sleep> backtrace:
.../abstract/connection_pool/reaper.rb:46:in `help! I'm stuck in some ruby code`
.../abstract/connection_pool/reaper.rb:46:in `sleep'
.../abstract/connection_pool/reaper.rb:46:in `block in spawn_thread'

#<Thread:0x000000010e530db8 .../drb/drb.rb:1555 sleep> backtrace:
.../lib/ruby/3.2.0/drb/drb.rb:986:in `select'
.../lib/ruby/3.2.0/drb/drb.rb:986:in `accept_or_shutdown'
.../lib/ruby/3.2.0/drb/unix.rb:106:in `accept'
.../lib/ruby/3.2.0/drb/drb.rb:1714:in `main_loop'
.../lib/ruby/3.2.0/drb/drb.rb:1557:in `block in run'
```

For me, this helped me to realize the test runner wasn't closing a connection
to the DB properly. A quick `brew services restart postgresql@14` fixed
everything, a solution I never would have fathomed to attempt without a
backtrace.

[1]: https://github.com/myronmarston
[2]: https://github.com/rspec/rspec-rails/issues/1353